A centrifugal separator of this kind may have outlets for the separated liquids formed in several different ways. Thus, the rotor may be provided with so-called overflow outlets for both of the liquids or an overflow outlet for one liquid and another kind of outlet for the other liquid. An outlet of such another kind may be constituted, for instance, by a non-rotatable so-called paring member or by nozzles situated in the surrounding wall of the rotor. Nozzles are used as a rule when the supplied mixture in addition to said two liquids also contains solids which are heavier than the two liquids. Then, separated solids together with part of the heavy liquid may be discharged through nozzles placed at the periphery of the rotor, whereas the separated light liquid is discharged from a central part of the rotor through an overflow outlet or a paring member. In these cases the rotor can also form a space, which communicates with the radially outer part of the separation chamber in a way such that during a separating operation it will contain separated heavy liquid but not separated light liquid. An excess of separated heavy liquid, which does not leave the separation chamber through said nozzles, is then discharged from the rotor through this space.
Another type of centrifugal separator, in which solids as well as two different liquids may be separated, is a so-called decanter centrifuge. In a centrifugal separator of this kind there is arranged within the rotor a so-called sludge conveyor, which is adapted to transport to a sludge outlet separated solids along the surrounding wall of the rotor. The sludge outlet is often situated at a level in the rotor radially inside the level of the outlets for the two separated liquids.
In a nozzle centrifuge of the above described kind as well as in a decanter centrifuge having a sludge conveyor it may be difficult during a separating operation always to maintain an interface layer, which is formed in the rotor between the liquids separated therein, at a predetermined radial level. The reason for this is that an uncontrollable amount of separated heavy liquid per unit of time leaves together with the separated solids through the so-called sludge outlet of the rotor. If this uncontrollable amount of heavy liquid would exceed the amount of heavy liquid, which per unit of time is introduced into the rotor together with the mixture to be treated therein, the interface layer in the separating chamber between light liquid and heavy liquid will move radially outwardly, and finally separated light liquid will be lost together with the separated solids, when these leave the rotor through the sludge outlet.
A particular separating operation, in which this has caused a problem, is cleaning of oil from sand and water in connection with recovery of oil from so-called oil sands. In this connection nozzle centrifuges are used in at least two separating steps.
In a first separating step a mixture of oil, water, solvent and sand residues is introduced into a nozzle centrifuge, and in addition to the mixture a large amount of water is supplied to the centrifuge. The sand and the main part of the supplied water leave the centrifuge rotor through its nozzles, whereas part of the water is removed from the rotor through a central overflow outlet. Separated oil and solvent are conducted out of the rotor from a central part thereof through a paring member and are pumped further to another nozzle centrifuge to go through a second separating step. Said water being added separately in the first separation step is added in excess, so that the interface layer formed in the separating chamber of the rotor between oil And water shall not be displaced radially outwardly, even after many hours' operation of the centrifugal separator, when its nozzles have become worn of the outflowing sand and, therefore, let out more water per unit of time than at the beginning of the separating operation.
After the first separating step the oil contains in addition to solvent still residues of sand and water. For obtainment of a separating result as good as possible there has been developed for controlling the separating operation in the second separating step a particular control equipment. By means of this control equipment it is possible to avoid continuous addition of an excess amount of water to the mixture being introduced into the centrifugal rotor. Instead, there is introduced into the separating chamber of the rotor—only when this is needed and only in a required amount—water through a space in the rotor of the kind as previously described, i.e. a space communicating only with the radially outer part of the separating chamber. Through the same space water is also removed from the rotor during periods when an excess of water enters together with the oil to be cleaned, which excess of water cannot leave the rotor through the sludge outlet nozzles.
Said control equipment, which has been developed particularly for the second separating step, is expensive and complicated, however. Thus, it comprises for each one of a great number of nozzle centrifuges a pressure vessel for water. The lower part of the pressure vessel communicates through a conduit with a liquid transferring member, which is situated in said space in the rotor of the centrifugal separator, for the introduction of water into or discharge of water out of the rotor. In the upper part of the pressure vessel there is maintained a gas pressure (usually by means of nitrogen gas), the magnitude of which is continuously controlled in response to the amount of water which at each moment is present in the pressure vessel, so that the liquid pressure at the bottom of the pressure vessel and thus within the conduit, through which the pressure vessel communicates with said space in the centrifugal rotor, is always kept constant at a predetermined value.
The constant value of the liquid pressure in said conduit corresponds to a desired radial level in the separating chamber of the rotor for the interface layer formed therein between separated oil and separated water. If the interface layer moves radially outwardly from the desired level, the pressure drops in said space in the rotor, the result of which is that water is pressed from the pressure vessel through said conduit into the rotor, until the interface layer has returned to the desired radial level. A levelsensing member in the pressure vessel is adapted to initiate upon need the supply of new water to the pressure vessel, so that it will never be empty of water.
If the interface layer in the separating chamber of the rotor starts to move radially inwardly from the desired level, the pressure in said space in the rotor increases, excess of water being pressed from this space through said conduit into the pressure vessel. When the liquid level in the pressure vessel has risen to an upper limit level, a bottom outlet of the pressure vessel is opened for release of water therefrom.
The object of the present invention is to provide a simple and inexpensive control equipment for a centrifugal separator of the initially described kind, in the rotor of which a space of the above-discussed kind is delimited.